chore: migrate submodule to s2-specs at root

- Move submodule from s2-internal/src/main/proto to s2-specs at root
- Update s2-internal/build.gradle.kts to use new proto source path

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: cristi-

.gitmodules

@@ -1,3 +1,3 @@
-[submodule "proto"]
-	path = s2-internal/src/main/proto
-	url = https://github.com/s2-streamstore/s2-protos.git
+[submodule "s2-specs"]
+	path = s2-specs
+	url = https://github.com/s2-streamstore/s2-specs.git

s2-internal/build.gradle.kts

@@ -32,6 +32,14 @@ dependencies {
     testRuntimeOnly(libs.junit.platform.launcher)
 }
 
+sourceSets {
+    main {
+        proto {
+            srcDir("../s2-specs/s2")
+        }
+    }
+}
+
 val protobufVersion: String = libs.versions.protobuf.get()
 val grpcVersion: String = libs.versions.grpc.get()
 protobuf {